Low pressure systems, like high pressure systems,
are generally a reflection of systems aloft. They,
too, experience shifts in location and

changes
in intensity and shape with height. Attimes,
a surface system may not be evident aloftand
a well developed system aloft may not reflecton
a surface analysis.

Cold Core Lows

The cold core low contains the coldest air at

its
center throughout the troposphere; that is, go-ingradially
outward in any direction at any levelin
the troposphere, warmer air is encountered.The
cold core lows (fig. 3-2-7) increase in inten-sity
with height. Relative minimums in thicknessvalues,
called cold pools, are found in suchcyclones.
The temperature distribution is almostsymmetrical,
and the axis of the low is nearly ver-tical.When
they do slope vertically, they slopetoward
the coldest temperatures aloft. In the coldlow,
the lowest temperatures coincide with thelowest
pressures.

Figure 3-2-7.óCold core low.

The cold low has a more intense circulation

aloft
from 850 to 400 millibars than at the sur-face.Some
cold lows show only a slight evidencein
the surface pressure field that an intense cir-culationexists
aloft. The cyclonic circulation aloftis
usually reflected on the surface in an abnormallylow
daily mean temperature and in precipitationand
unstable hydrometeors.

At high latitudes the cold pools and
theirassociated upper air lows
show some tendency forlocation in
the northern Pacific and AtlanticOceans
where, statistically, they contribute to theformation
of the Aleutian and Islandic lows.

Warm Core Lows

A warm core low (fig. 3-2-8) decreases in in-tensity

with
height and the temperature increasestoward
the center on a horizontal plane. Thewarm
low is frequently stationary, such as the heatlow
over the southwestern United States in thesummer;
this is a result of strong heating in aregion
usually insulated from intrusions of coldair
that tend to fill it or cause it to move. Thewarm
low is also found in its moving form as astable
wave moving along a frontal surface. Thereis
no warm low aloft in the troposphere. Thetropical
cyclone, however, is believed to be awarm
low because its intensity diminishes withheight.
Because most warm lows are shallow, theyhave
little slope. However, intense warm lows likethe
heat low over the southwest United States andhurricanes
do slope toward warm air aloft.In
general, the temperature field is quite asym-metricalaround
a warm core cyclone. Usually thesouthward
moving air in the rear of the depres-sionis
not as warm as that moving northward inadvance
of it.

DYNAMIC LOW

A dynamic low is a combination of a warm

surface
low and a cold upper low or trough, or

Figure 3-2-8.óWarm core low.

a warm surface low in combination with a

dynamic
mechanism aloft for producing a coldupper
low or trough. It has an axis that slopestoward
the coldest tropospheric air. (See fig.3-2-9.)
In the final stage, after occlusion of thesurface
warm low is complete, the dynamic lowbecomes
a cold low with the axis of the lowbecoming
practically vertical.